Remove single-player mode to focus on multiplayer

Removed all single-player/offline mode functionality:
- Removed ~2,200 lines of SQLite database code
- Removed 11 public SP methods from GameHandler
- Removed SP member variables and state flags
- Removed SP UI elements (auth screen button, game settings)
- Removed SQLite3 build dependency
- Deleted docs/single-player.md
- Updated documentation (README, FEATURES, CHANGELOG)

Files modified:
- src/game/game_handler.cpp: 2,852 lines (down from 4,921)
- include/game/game_handler.hpp: Removed SP API
- src/core/application.cpp/hpp: Removed startSinglePlayer()
- src/ui/*: Removed SP UI logic
- CMakeLists.txt: Removed SQLite3

All online multiplayer features preserved and tested.

src/ui/character_screen.cpp

@@ -154,10 +154,8 @@ void CharacterScreen::render(game::GameHandler& gameHandler) {
                 ss << "Entering world with " << character.name << "...";
                 setStatus(ss.str());
 
-                // Only send CMSG_PLAYER_LOGIN in online mode
-                if (!gameHandler.isSinglePlayerMode()) {
-                    gameHandler.selectCharacter(character.guid);
-                }
+                // Send CMSG_PLAYER_LOGIN to server
+                gameHandler.selectCharacter(character.guid);
 
                 // Call callback
                 if (onCharacterSelected) {